timers: added os::Timer::waitFor(...) and os::Timer::waitForUntil(...) member functions

This is needed for the implementation of `OCL::TimerComponent::wait()` and `OCL::TimerComponent::waitFor()`.
See https://github.com/orocos-toolchain/ocl/issues/23.

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>